The present invention relates to the preparation of aqueous, cosolvent-free polyurethane (“PU”) dispersions with blocked isocyanate groups, to baking varnishes prepared from them, and to their use in coatings.
Recent years have seen a sharp rise in the profile of aqueous paints and coating materials in the wake of increasingly stringent emissions directives governing the solvents released during paint application. Although for many fields of application there are now aqueous coating systems available, these systems are often unable to attain the high quality level of conventional, solventborne coating materials with respect to solvent and chemical resistance, elasticity and mechanical durability.
Even waterborne paint systems, based for example on aqueous dispersions, frequently still contain cosolvents. For PU dispersions it is generally not possible to avoid these solvents, since the preparation of corresponding dispersions via prepolymers often necessitates cosolvents. In addition, in the absence of cosolvents, the PU dispersions and the paint formulations frequently lack storage stability. The solvent N-methylpyrrolidone (“NMP”) in particular is in widespread use in the field of aqueous dispersions and paints. Examples are the polyisocyanate crosslinker dispersions with blocked isocyanate groups that are described in EP-A 0566953 or in EP-A 0942023. The crosslinker dispersions and the paints described in both references contain NMP as cosolvent.
The solvent NMP is suitable for preparing a large number of PU dispersions, but is classified as teratogenic by the European Union. As a consequence of this, this solvent ought to be avoided as much as possible.
The object of the invention is to provide cosolvent-free, storage-stable polyisocyanate dispersions with blocked isocyanate groups.